The invention relates to a method for plasmas-assisted reactive electron beam vaporization and, in particular, high rate vaporization, preferably for the production of wear-resistant, hard layers and barrier layers. Such layers consist, e.g., of metal oxides and serve as scratch and wear protecting layers on glass, plastic or other materials. They are, inter alia, used for facade glass and in the building industry. The barrier action of the layers is particularly significant in the packaging industry in order to make the material, e.g., oxygen and water vapor-impermeable. Barrier layers are also important for corrosion protection and are used in this connection.
A basic prerequisite for the production of the layer on a technical scale is to ensure constant characteristics thereof throughout the coating time. This condition makes high demands on the layer vaporization method, because generally use is made of highly productive installations with a high travel speed and large substrate surface. Under high rate coating conditions, process parameter fluctuations are unavoidable.
In the case of electron beam vaporization, it is known to monitor certain coating or layer characteristics throughout the vaporization period. Use is made for this purpose of optical characteristics, e.g. transmission and reflection, and also the electrical characteristics, such as conductivity, which can be measured in situ. It is known in this connection for the coated film, immediately following the vaporisation process, to pass through a measuring roller system. The measuring roller system determines the layer thickness distribution by measuring the electrical conductance. By regulating the residence times of the electron beam at different points of the material being vaporised, the layer thickness distribution is adjusted in a planned manner (see S. Schiller et al, Proceedings of the 2nd Int. Conference on Vacuum Web Coating, Fort Lauderdale, Fla. U.S.A., October 1988).
It is also known, for determining the optical characteristics, to establish the transmission and/or reflection of optical layers in the wavelength range which is of interest and to calculate therefrom the color locus or other applicative, optical characteristics. Another standard method is to determine the layer thickness in situ from the transparency or reflection (see G. Whitehead, P. Grant, "The Optical Monitor", Proc. 28th Ann. Techn. Conf. Soc. Vac. Coaters, Philadelphia, 1985, pp.109-115). The refractive index can be determined from the measurement of the reflection and absorption spectra. For the aforementioned fields of use of the materials or substrates to be coated, significance is attached to the mechanical characteristics such as hardness, abrasion and barrier action. Numerous measuring methods exist for measuring in a direct manner such characteristics (see K. Nitzsche, "Schichtmesstechnik", VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig, 1974). However, such methods are not suitable for in situ measurement purposes.
The mechanical characteristics are largely dependent on the internal layer structure and composition. In the case of high rate electron beam vaporization, there is no constancy of the indicated characteristics due to the inherent, complex dependence on the deposition conditions. The structure and composition of the deposited layers must therefore be measured and regulated in situ, at least in a direct manner, in order to maintain constant the requisite, mechanical characteristics.
There is therefore needed a method for plasma-assisted reactive electron beam vaporization with which it is possible to measure in situ certain non-optical layer characteristics and, in particular, mechanical characteristics, so as to be able to reproducibly apply the layers in a highly productive manner and with high a constancy. The values obtained in a known manner for the reflection and/or transmission and optical absorption at wavelengths of 150 to 800 nm must also be processed as a regulating signal. The method must in particular be suitable for continuous flow units for producing wear-resistant, hard layers or layers having a barrier action. Measured quantities are to be found which can be non-destructively determined in situ. For process control purposes, use is to be made of suitable and, optionally, also known regulating and control possibilities.
According to the present invention, these needs are met by a method for plasma-assisted reactive electron beam coating by vaporization of substrates with hard, wear-resistant layers and/or layers with barrier characteristics by producing a controllable plasma in the process zone between the vaporizing material and the substrate to be coated and the in situ measurement of optical characteristics. Immediately after the substrate has passed through the vaporizing zone, the reflection and/or transmission and the absorption capacity are measured in the wavelength range .DELTA..sub.k 150 nm to 800 nm. From this measurement, the refractive index n and the optical absorption coefficient k are separately determined. These values are compared with an experimentally determined desired value. With the control signal obtained therefrom, and in the case of a constant reactive gas partial pressure, the parameters of the plasma are controlled in such a manner that the initially measured optical characteristics of the vapor-deposited layer are kept constant.
It has surprisingly been found that certain non-optical characteristics, such as, e.g., the hardness, abrasion and barrier action of layers correlate with certain optical characteristics, such as absorption, reflection and transmission. Clearly, this dependence is due to the fact that changes in the packing density, the chemical composition and the binding states act in the same direction in mechanical and barrier characteristics and also in optical characteristics. It has been found that the tolerance limits of the mechanical and barrier characteristics are always respected if the optical characteristics are kept constant within certain tolerance limits. For this purpose, the refractive index n and absorption coefficient k are monitored. It has also been found that in the case of such plasma-assisted reactive processes apart from many other parameters, which must be experimentally determined and kept constant, the reactive gas partial pressures and plasma parameters have a considerable influence on these characteristics. An important part, in connection with the layer formation process, is played by the reactivity and formation enthalpy of the chemical compound, as well as the dissociation capacity in the case of electron beam vaporization. It is fundamentally possible to regulate the reactive gas partial pressure or plasma parameters as a function of the in situ measured values n and k in order to obtain the layer characteristics. It has been found that the effectiveness of the selected regulating or control process is dependent on the layer system. Thus, e.g., when depositing SiO.sub.x layers, a regulation of the partial pressure of the reactive gas is particularly effective for obtaining constant barrier characteristics. However, in the deposition of aluminum oxide, the regulation of the plasma parameters has proved advantageous in order to ensure the necessary mechanical properties, such as hardness and abrasion.
The determination of the refractive index n and the absorption coefficient k is, however, not technically possible in the vicinity of the plasma area and therefore in a direct manner during layer formation in the case of high rate vaporization. This naturally leads to a time lag between layer formation and the determination of n and k. Thus, as a result of this time lag, a given basic stability of the entire vaporization process is necessary in order to ensure that the layer characteristics do not exceed the requisite tolerances. The necessary plasma excitation can take place in a known manner inter alia by ion sources or by electron impact-determined low pressure plasmas. The prerequisite for an effective control of the reactive gas partial pressure is the keeping constant of the plasma excitation during the coating process. Various control methods are known for keeping the plasma parameters constant and are based on the measurement of the plasma parameters, e.g. by means of probes or optical emission spectroscopy. For solving the present problem, the measurement of the optical plasma emission has proved particularly advantageous.
When regulating the plasma parameters as a function of n and k, the reactive gas partial pressure must be kept constant. For this, it is possible to regulate the reactive gas flow or the suction capacity of the pumps as a function of the reactive gas partial pressure.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.